The present invention relates to a liquid crystal display apparatus, and in particular, to the apparatus capable of curbing image quality degradation due to crosstalk and implementing high image quality and reduction of driving voltage and power consumption.
The liquid crystal display apparatus is widely used as a display portion of mobile equipment represented by a notebook PC and a portable telephone taking advantage of its characteristics of low profile, light weight and super low power consumption, and is beginning to be widespread as a monitor for a desktop PC and a liquid crystal television because it shows performance superior to the display apparatuses such as a CRT (Cathode Ray Tube) in-terms of low profile, light weight, high-resolution display and so on.
As for a display principle of the liquid crystal display, an IPS mode (In-plane Switching Mode) characterized by a wide viewing angle, MVA (Multi Domain Vertical Alignment), OCB (Optically Compensated Birefringence) and so on are used in addition to mainstream TN (Twisted Nematic).
Among these display modes, the in-plane switching mode has an excellent viewing angle characteristic and is capable of gathering on one of the substrates almost all the components of a liquid crystal panel such as active elements, electrodes and a color filter in addition, and so it is expected as a mode capable of inexpensively implementing a high-resolution liquid crystal display apparatus having no problem of electrode pattern alignment between the upper and lower substrates.
As for a challenge in an early development stage of the in-plane switching mode, there is improvement in the image quality degradation due to the crosstalk from a row line for applying a signal voltage based on image data. It occurs, when the row line and a pixel electrode are closely positioned, because variation of the signal voltage is overlapd at a certain ratio on a pixel potential as a crosstalk voltage due to a coupling capacitance between the line and the electrode. In order to prevent it, a configuration for using a second pixel electrode also as a screening electrode was devised, and is disclosed in JP-A-6-202127 specification.
Another challenge of the in-plane switching mode is that, as a liquid crystal is driven by a horizontal electric field, the driving voltage is apt to rise when an inter-electrode distance is widened to increase an opening ratio. If the liquid crystal driving voltage becomes high, it is necessary to enhance dielectric strength of a driving element placed around the liquid crystal panel for the sake of applying a voltage to the liquid crystal, and it is also necessary, in the case where the driving element is comprised of active elements such as thin film transistors formed by polysilicon or amorphous silicon of a low temperature process on a glass substrate, to enhance the dielectric strength of the thin film transistors. In addition, the dielectric strength required for the active elements in the pixel portion also becomes high. Due to enhancement of the dielectric strength of the active elements, there is a danger of damaging some of the characteristics of the liquid crystal display apparatus such as limitation of higher resolution due to increased area and increase in non-display area of the driving element, increase in the process of enhancing the dielectric strength of the thin film transistors, and increase in glass periphery area.
The authors hereof devised a differential driving mode, as the configuration capable of applying a sufficient voltage to the liquid crystal and also reducing the voltage of the dielectric strength of the driving element even with a decreased number of lines, wherein a plurality of TFTs are placed in one pixel and the liquid crystal is driven by a differential of the voltages written by these TFTs to display an image, which mode was disclosed in JP-A-6-148596 and JP-A-6-202073 specifications.
In order to more clarify an object of the present invention, a basic configuration common among all the active matrix type liquid crystal display modes and a driving mode thereof will be described first, and then the object of an active matrix type liquid crystal display apparatus will be described by taking the in-plane switching mode as an example.
FIG. 22 shows an equivalent circuit diagram of the active matrix type liquid crystal display apparatus. At a start of a selected period, the potential for rendering an active element 203 on is given to a row line 201 by a gate driver 106, the potential based on image data is given to a row line 202 by a gate driver 107, and the potential based on the image data is given to a pixel electrode 210 via the active element 203. A liquid crystal 208 and a holding capacitance 205 connected in parallel are charged by a potential difference between the potential of the pixel electrode 210 and that of a common line 209 to a second pixel electrode 204 and the holding capacitance 205 of the liquid crystal. In the cases of having on the opposite substrate side a second pixel electrode which is plane and common among all the pixels as in the TN mode, the MVA mode or the OCB mode, the second pixel electrode 204 is normally formed on the opposite substrate side and the common line 209 of the holding capacitance 205 is formed on the same substrate as the active element as shown in this drawing. On the other hand, in the configuration such as the in-plane switching mode capable of forming the pixel electrode 210 and the second pixel electrode 204 on the same substrate, it is possible to connect the common line 209 to the second pixel electrode 204. At the end of the selected period, the potential for rendering the active element 203 off is given to the row line 201, so that the writing is completed. Charging of the liquid crystal 208 and the holding capacitance 205 is finished in a very short time compared with an optical response of the liquid crystal. At this time, transmittance shown by the liquid crystal 208 corresponds to an absolute value of the voltage which is written and is not dependent on polarity of the voltage.
The crosstalk in the active matrix type liquid crystal display apparatus in the in-plane switching mode will be described by referring to FIG. 21.
FIG. 21 shows an equivalent circuit of the active matrix type in-plane switching mode, and FIG. 23 shows a plane layout diagram thereof. The overall configuration as the liquid crystal display apparatus is omitted since it is the same as FIG. 22. This drawing is the equivalent circuit diagram and at the same time, it shows a placement which is almost plane, where the second pixel electrode 204 is normally connected to the common line 209 of a constant potential so that the second pixel electrode 204 acts as an electrical shield against voltage variation of Vd1 and Vd2 of the row line 202 so as to stabilize the potential of the pixel electrode 210. Accordingly, it is possible, by taking sufficient width of the second pixel electrode 204, to be hardly influenced by potential variation of the row line 202 so as to display a high-quality image without crosstalk. However, if the width of the second pixel electrode 204 is rendered smaller for the purpose of increasing the opening ratio or in the case of small pixel area due to high resolution, the voltage variation of the row line 202 to which the voltage based on the image data is applied is transferred via capacitance coupling of a parasitic capacitance 631, so that crosstalk voltage is overlapd on the pixel electrode 210. In this case, there are the cases where, in the in-plane switching mode that is a normally black mode, contrast reduction due to rise in black display luminance, the crosstalk in a row direction in half-tone display, or a flicker or an afterimage due to superimposition of asymmetrical voltage by the polarity may be observed.
Of such image quality degradation, a curbing method by a driving mode has been adopted as to the flicker and the crosstalk in a row direction in half-tone display. Here, the flicker, the polarity of the voltage to be given to the liquid crystal and the method of curbing the flicker will be described. It is generally known that the liquid crystal has its characteristics degraded by applying DC voltage thereto, and so the image data given to the liquid crystal of certain pixels is normally applied by reversing its polarity at least for each frame. While the transmittance shown by the liquid crystal is determined by the size of the applied voltage and is not dependent on polarity thereof, the crosstalk is generated, if driven by using the active element, due to the parasitic capacitance which the active element has or a leakage current when the active element is off so that, even if the voltage is supplied from a data driver so as to apply the voltage of the same size to the first pixel electrode 210, there is a slight deviation to a voltage value actually applied to the liquid crystal depending on its polarity. Normally, the liquid crystal display apparatus displays one frame at 60 Hz. If the voltages applied to the liquid crystal at the positive and negative polarities are equal, human eyes cannot observe it as the flicker since it is an AC drive of 60 Hz, whereas the same image data is recognized as the flicker of a 30 Hz component if the luminance is different between the positive and negative polarities. As for a method of curbing the flicker, it becomes impossible, by increasing a frame frequency and displaying it at 120 Hz for instance, to recognize the flicker due to the difference in the luminance between the positive and negative polarities since it exceeds the frequency discriminable by the human eyes. As for another method of curbing the flicker, it is also possible, by spatially distributing the pixels to be written at the positive polarity and those to be written at the negative polarity, to render the difference in the luminance average so as not to have it recognized by the human eyes.
In the past, the method of spatially distributing the writing polarities was exclusively used, especially for a large liquid crystal display apparatus, in order to avoid limitation of driving ability of a gate driver and the data driver. The following four methods of driving the active matrix type liquid crystal display apparatus, including a method of not spatially distributing, are known.
(1) Frame inversion driving: It is the driving mode of reversing the polarities for each frame without spatially distributing the polarities of the applied voltage, which is the easiest way to observe the flicker.
(2) Per-line inversion driving: It is the driving mode of reversing the polarities of the applied voltage for each line, and further reversing the polarities for each frame.
(3) Per-row inversion driving: It is the driving mode of reversing the polarities of the applied voltage for each row, and further reversing the polarities for each frame.
(4) Dot inversion driving: It is the driving mode of reversing the polarities of the applied voltage both for each line and for each row, and further reversing the polarities for each frame, which is the best way to curb the flicker.
As the frame inversion driving writes the image data of the same polarity on the entire screen, it has an advantage that the potential to be outputted in a certain frame by the data driver can always have the same polarity as the second pixel electrode and it can be combined with a common AC driving mode for varying the potential of the second pixel electrode according to the writing polarity so as to use the data driver of low pressure-resistance. However, in the case where the polarity of a displayed image to be visualized is reversed for each frame at the frame frequency of 60 Hz in the past pixel configuration, the flicker can be easily recognized due to the aforementioned difference in the writing characteristics of the positive and negative polarities.
The per-line inversion driving and the per-row inversion driving distribute the polarities of the displayed image in the screen, and display it by rendering the difference in the luminance due to the different polarities average to the human eyes so that the flicker cannot be recognized.
The dot inversion driving is a driving mode whereby, as it reverses the polarities of the displayed image for each line and further for each row, the difference in the luminance due to the different polarities is further rendered average so as to prevent the flicker from being recognized.
Of the above four types of driving mode, the per-line inversion driving mode and the dot inversion driving mode have the writing polarity changed for each line, and so the average of the voltages applied on the row line, not by the image, is almost constant so that they are capable of significantly curbing the crosstalk voltage applied to the pixel electrode from the row line via the parasitic capacitance. However, they are not all-round driving modes since there is a display pattern for canceling the effects of these driving modes, such as the pattern of repeating every other line. Furthermore, the per-line inversion driving mode and the dot inversion driving mode have a number of constraints to various low-voltage driving modes. For instance, a common line configuration capable of combination with the dot inversion driving mode in the low-voltage driving mode for rendering the common line as AC has not been implemented. In addition, it is considered that there is a constraint to a resistance value in the low-voltage driving in the per-line inversion driving mode and so the high image quality can hardly be obtained with higher resolution and on a larger screen.
Next, the differential driving mode will be described by referring to FIG. 20. To briefly describe this pixel configuration and the driving mode, two active elements are provided to one pixel, and the voltage is applied to the liquid crystal by the difference voltage between the potential of the adjacent row line and the row line of the pixel to which the signal voltage based on the image data is applied, where it is possible, as a characteristic of this mode, to implement a high opening ratio since there is no need to provide the common line specific thereto. FIG. 20 shows the pixel configuration of the method of writing the voltage to the liquid crystal by the potential difference between the row lines on both sides sandwiching the pixel electrode. While any low-voltage driving including the dot inversion driving is possible by this method since the potential difference between the adjacent row lines is arbitrarily made, the potential difference between the adjacent lines is gradually overlapd so that the driving element of high dielectric strength becomes necessary or a complicated row line potential must be set.
An object of the present invention in consideration of the above is to implement low-voltage driving for a number of driving modes mentioned above with a high image quality that is crosstalk-free.
Another object of the present invention is to provide a bright liquid crystal display apparatus which has a small number of lines and a high opening ratio.
According to an embodiment of the present invention, the liquid crystal display apparatus has a pair of substrates of which at least one is transparent and a liquid crystal layer supported between the pair of substrates, wherein one of the pair has a plurality of row lines, a plurality of row lines placed intersecting the plurality of row lines and a plurality of common lines, and a first active element is provided near the intersection of the plurality of row lines and the plurality of row lines, and a voltage according to image data is written to pixels placed like a matrix through the first active element, and the above described apparatus has a second active element, a first pixel electrode and a second pixel electrode provided in the pixel, and has one terminal of the first active element connected to the first pixel electrode, the other terminal to the row line, one terminal of the second active element to the second pixel electrode, the other terminal to the common line or the row line not involved in writing of the pixel electrode, and besides, the first and second active elements are brought into conduction during a period of writing the voltage to a liquid crystal, and the first and second active elements are brought into a high resistance state during a holding period, and in addition, a capacitance between the row line and the first pixel electrode and that between the row line and the second pixel electrode are equal as to each row line adjacent to the pixel.
According to another embodiment of the present invention, the liquid crystal display apparatus has the pair of substrates of which at least one is transparent and the liquid crystal layer supported between the pair of substrates, wherein one of the pair has the plurality of row lines, the plurality of row lines placed intersecting the plurality of row lines and the plurality of common lines, and the first active element is provided near the intersection of the plurality of row lines and the plurality of row lines, and the voltage according to the image data is written to the pixels placed like a matrix through the first active element, and the above described apparatus has the second active element, the first pixel electrode and the second pixel electrode provided in the pixel, and has one terminal of the first active element connected to the first pixel electrode, the other terminal to the row line, one terminal of the second active element to the second pixel electrode, the other terminal to the common line, and besides, the first and second active elements are brought into conduction during the period of writing the voltage to the liquid crystal, and the first and second active elements are brought into the high resistance state during the holding period, and in addition, a superimposing portion via a dielectric film of the first pixel electrode and the second pixel electrode is provided around the center in the row direction of the pixels, and moreover, the first pixel electrode and the second pixel electrode are formed to be in an axisymmetric relation in the row direction centering on the superimposing portion.
Moreover, in these embodiments, a holding capacitance is formed in the superimposing portion of the first and second pixel electrodes.
Furthermore, the common line is placed almost in parallel with the row line, and a projection-like shielding electrode is provided in part of the common line, so that the shielding electrode sandwiches the row line or is positioned between the row line and the first and second pixel electrodes or placed to cover the row line. A further improvement in the image quality due to this configuration can be expected.
According to a further embodiment of the present invention, the liquid crystal display apparatus has the pair of substrates of which at least one is transparent and the liquid crystal layer supported between the pair of substrates, wherein one of the pair has a plurality of row lines and a plurality of row lines, and the pixels are placed corresponding to intersections of the plurality of row lines and the plurality of row lines, and the above described apparatus has the first pixel electrode and the second pixel electrode provided in the pixel, and has the first active element placed in the pixel and has the output terminal thereof connected to one row line adjacent to the first pixel electrode and pixel, and has the second active element placed in the pixel and has its output connected to the other row line adjacent to the second pixel electrode, and besides, the first and second active elements are brought into conduction during the period of writing the voltage to the liquid crystal, and the first and second active elements are brought into a high resistance state during the holding period, and the difference voltage between the adjacent row lines is applied to the liquid crystal to display the image, and in addition, the capacitance between the row line and the first pixel electrode and that between the row line and the second pixel electrode are equal as to each row line adjacent to the pixel.
As for the form of the pixels, for the sake of the most effective workings, the superimposing portion via the dielectric film of the first pixel electrode and the second pixel electrode is provided around the center in the row direction of the pixels, and the holding capacitance is formed in the superimposing portion, so that the first pixel electrode and the second pixel electrode are formed to be in the axisymmetric relation in the row direction centering on the superimposing portion.
As for the driving mode, in order to achieve it, the pixels comprising one line are divided into two pixel groups, and two row lines are provided to one line so that each row line controls the writing of a predetermined pixel group.
In addition, in order to achieve it, the first and second active elements are comprised of two types of active elements P and N, and each pixel is comprised of the active elements of the same type so that the type of the active element to be connected to each row line is different in each adjacent pixel.
Moreover, as for the driving mode for improving the image quality, it can be implemented by rendering an average of the voltages written to the first and second pixel electrodes comprising the pixel constantly fixed.
Other objects, features and advantages of the invention will become apparent from the following description of the embodiments of the invention taken in conjunction with the accompanying drawings.